Occupancy sensors are used in a variety of applications to detect the presence of people and variously turn on lights, ventilation equipment etc. It is known to link the detectors together with other equipment in a network to allow communication of information between sensors and, for example, output devices. Furthermore, there exist networks which allow sensors to communicate wirelessly.
There are three main types of occupancy sensor on the market: PIR (passive infrared) which rely on body heat to detect occupancy; ultrasonic sensors measure reflected ultrasound; and microwave sensors which use reflected radio waves and detect Doppler shifts caused by moving targets.
There are some specific problems associated with combining microwave occupancy sensors and wireless data transceivers in the same product. The microwave occupancy sensor consists of a transmitter, a receiver and a mixer. The transmitter emits radiation at a given frequency. A high sensitivity receiver receives the signal reflected from the targets and mixes it with the outgoing signal to generate an IF (intermediate frequency) output which is analysed for Doppler shift. The microwave sensor's receiver is very sensitive, capable of detecting very small amounts of in-band energy, i.e. close to the operating frequency. Due to its high sensitivity the microwave receiver is susceptible to interference from powerful out-of-band signals. Even though the wireless data transmitter does not operate at the same frequency as the microwave detector, interference arises where a small part of the powerful data transmitter's signal is admitted to the circuitry of the microwave detector. Further, the microwave receiver is also very sensitive to power supply fluctuations within the circuit, which occur when wireless data is transmitted.
This invention, at least in its preferred embodiments, seeks to provide a method for combining wireless data transmission and microwave occupancy sensing in the same product, overcoming the various sources of false triggers.